1. Field of the Invention
The subject preamplifier system is generally directed to a system for amplifying and conditioning an input electromagnetic signal for insertion into a high power amplifier. More specifically, the subject preamplifier system is directed to a system employing multi-cell silicon carbide transistor assembly devices to preamplify an RF signal with a relatively high gain while substantially preserving operational linearity.
Factors such as high power density--that is, the ability to yield high power levels for a given device size and weight--along with factors such as the degree of voltage and temperature tolerance are quite critical in many electrical applications, particularly in high power applications. The capacity to tolerate higher operating voltages, for instance, enables an active electrical device to operate with lower currents, hence with greater efficiency. Similarly, the capacity to tolerate higher operational temperatures generally enables an active electrical device to yield greater power while diminishing the likelihood of irreparable damage to the device when subjected to a given set of operating conditions. Indeed, temperature tolerance is not only a significant limiting factor to the safe operating point of a given electrical device, it is a significant limiting factor to the space density with which a plurality of such devices may be physically disposed within a space allocated therefor without necessitating overly elaborate cooling measures.
Another factor of considerable concern in many applications is the system's operational frequency bandwidth. Currently, in RF broadcast applications such as transmission within the UHF TV frequency range, known systems invariably offer operational bandwidths not substantially wider than one transmission channel, which is approximately 6 MHz wide. A broadcast system typically employs a plurality of transmitter modules; hence, each module must then be individually tuned for the particular transmit channel to be utilized. The need for such tedious tuning would be obviated were there available for use system modules offering operational frequency bandwidths wide enough to encompass a plurality of transmission channels without significant compromise of their operation in other respects. Any tuning of the system modules beyond the course pre-tuning performed during their manufacture would be unnecessary so long as system modules pre-tuned to a bandwidth encompassing the desired transmission channel are employed.
Another parameter of notable concern in RF broadcast applications is the linearity with which at least certain portions of the given system operate. It is highly desirable in such systems as a preamplifier system that the response characteristic of the amplification section therein remain stable for the full range of possible input signals. It is further desirable that this linearity of operation be preserved while maintaining optimal levels of power density and bandwidth.
2. Prior Art
Preamplifier systems employing cascaded amplifying stages are known in the art. Silicon carbide transistor devices are also known in the art. The best prior art known to Applicants includes U.S. Pat. Nos. 4,631,492; 4,670,789; 4,710,815; 4,800,426; 5,134,485; 5,686,737; 5,270,554; and, 5,726,605. The systems known in the prior art, however, do not include a preamplifier system which yields the combined degree of power density, voltage and temperature tolerance, wide bandwidth, and operational linearity yielded by the subject preamplifier system.
Silicon-based active electrical devices which are prevalent in the art do not offer the power density and voltage/temperature tolerance offered by silicon carbide transistor devices. Consequently, silicon carbide transistor devices have been utilized in the art in numerous applications requiring high peak power, pulsed operation of the active device. Such devices were, therefore, invariably operated in either Class B or Class C operational modes. There is no system heretofore known incorporating silicon carbide transistor devices wherein substantially linear operation thereof is realized, much less a system wherein such linear operation accompanied by an operational frequency bandwidth great enough to encompass a plurality of 6 MHz-video transmission channels is realized.